GAZA CITY, gaza strip — The conflict in Gaza raged Sunday, even as President Barack Obama pressed Israel for an “immediate, unconditional humanitarian cease-fire,” reflecting concern over the rising death toll.

Israel denied Sunday that it was responsible for one of the most shocking attacks in the 20-day conflict, saying its soldiers were not behind the deaths of 16 Palestinians killed by shelling three days earlier in a U.N. school near an area of intense fighting.

Israel’s military, however, offered no details about what it called a “comprehensive inquiry” into the incident. It released a grainy, 26-second aerial video showing what the military called an “errant” mortar landing in an empty courtyard of the school as proof that it didn’t kill anyone.

The U.N. Relief and Works Agency, which used the school as a shelter for Palestinians fleeing the violence, did not accept Israel’s conclusion.

In a telephone conversation with Israeli Prime Minister Benjamin Netanyahu on Sunday, Obama reaffirmed Israel’s right to defend itself and condemned Hamas attacks, the White House said in a statement.

But as the administration tried to balance its support for Israel with criticism of civilian casualties in Gaza, Obama also urged an immediate humanitarian cease-fire.

He expressed “the United States’ serious and growing concern about the rising number of Palestinian civilian deaths and the loss of Israeli lives.”

The current conflict has killed more than 1,035 Palestinians, more than 70 percent of them civilians, according to the United Nations. Israel has lost 43 soldiers, the largest toll since its 2006 war with Lebanon. Hamas mortar and rocket attacks from Gaza have killed two Israeli civilians and a Thai worker within Israel.

Netanyahu, in appearances on American Sunday morning talk shows, signaled that he planned to keep targeting Palestinian militants and destroying Hamas tunnel networks, through which the fighters have sought to infiltrate Israel.

“Israel is not obliged and is not going to let a terrorist organization determine when it’s convenient for them to fire at our cities, at our people, and when it’s not,” Netanyahu said on “Fox News Sunday.”

Israel resumed airstrikes in the Gaza Strip mid-morning Sunday, after offering Saturday night to extend a humanitarian cease-fire. Hamas rejected the Israeli offer to extend the truce for 24 hours. Rockets flew from Gaza into Israel throughout the morning, with six reaching Tel Aviv. Israeli artillery and airstrikes pounded parts of Gaza.

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